1. Field of the Invention
The present invention relates to an electric double layer capacitor that comprises an electrode unit having a pair of polarizable electrodes and a separator interposed between the pair of polarizable electrodes, the pair of polarizable electrodes each containing a carbon material, and in which the pair of polarizable electrodes is impregnated with an electrolyte solution. The invention also relates to a method of manufacturing the capacitor.
2. Description of Related Art
A well-known example of electric double layer capacitor is a coin-shaped electric double layer capacitor having the following configuration. The coin-shaped electric double layer capacitor comprises an element having a pair of polarizable electrodes, each composed mainly of a carbon material, and a porous separator interposed between the pair of polarizable electrodes. The element is accommodated in a case composed of a pair of current collector metal foils and an insulative resin for insulating the current collector metal foils.
Conventionally, the polarizable electrodes have been made of a porous carbon material, such as activated carbon and fibrous activated carbon. The electric double layer capacitor using the porous carbon material for the polarizable electrodes shows large internal resistance and is therefore unable to deliver a large current; in other words, it has a drawback of poor high-rate performance.
In addition, the porous carbon material needs to be fabricated through complicated manufacturing processes such as an alkali activation process and a steam activation process, necessitating higher manufacturing costs. Furthermore, the specific surface area of the porous carbon material almost reaches its limit at 3200 m2/g, and it is difficult to make the specific surface area higher than the limit.
In view of these problems, various proposals have been made. For example, Japanese Published Unexamined Patent Application No. H10-97956 proposes use of fullerene as a material for the polarizable electrodes. Japanese Published Unexamined Patent Application No. 2000-124079 proposes that the material for the polarizable electrodes contain a carbon nanotube. The fullerene and the carbon nanotube are obtained industrially through a physical growing process, such as arc discharge. Use of the fullerene and the carbon nanotube as a material for the polarizable electrodes achieves an improvement in the capacitor's high-rate performance compared to the use of the porous carbon materials, such as activated carbon and fibrous activated carbon.
A problem with the foregoing conventional techniques, however, has been that when the fullerene is subjected to an alkali activation process or a steam activation process for a long period of time or the carbon nanotube is subjected to an alkali activation process for a long period of time for the purpose of improving the capacity of the electric double layer capacitor further, molecular structures of the fullerene and the carbon nanotube are destroyed, and thus, further increase in the capacity has not been possible.